1. Technical Field
This invention relates to the technology of castable aluminum alloys, and more particularly to such alloys for service at high temperatures such as in the range of 400.degree.-600.degree. F.
2. Discussion of the Prior Art
Traditional aluminum casting alloys (i.e., SAE 332 or 390) are strengthened by additions of one or more of the following alloying elements: Cu, Mg, Si, Ni, and Zn. These elements strengthen the alloy by solid solution strengthening and by precipitation or dispersion strengthening mechanisms involving components such as CuAl.sub.2, Mg.sub.2 Si, MgZn.sub.2, and Al.sub.2 CuMg. For example, in SAE 332, consisting of about 8.5-10.5% Si, 2.0-4.0% Cu, 0.5-1.5% Mg, 1.2% Fe max., 0.50% Mn max., 1.0% Zn max., 0.50% Ni max., and 0.25% Ti, compounds are formed which have been identified as CuAl.sub.2, Mg.sub.2 Si, and Al.sub.2 CuMg. The SAE 332 alloy, a widely used piston alloy, has an ultimate tensile strength (UTS) property at 400.degree. F. after 1/3 hour of only 23 ksi, a yield strength (YS) of 17 ksi, and an elongation of about 1%. It is known that the effectiveness of the major strengthening mechanism in most of the aluminum alloys decreases with time as the alloy is exposed to temperatures over about 350.degree. F. Therefore, the difference observed after short-term exposure will become more pronounced with time and, after 1200 hours, will be about 19 ksi for UTS, 14 ksi for YS, and 1.5% for elongation. At 500.degree. F., these properties become considerably worse, such as after 1000 hours, a UTS of 16 ksi, and a YS of 10 ksi. Similar restricted high temperature properties are found with the wear resistant engine bore material 390, after 1000 hours at 500.degree. F., such as 10 ksi for UTS.
Conventional aluminum casting alloys have been used to the limits of their properties in many cases and are unable to meet a long-felt need for aluminum casting alloys having higher strength at temperatures of 400.degree. F. or higher, particularly for automobile applications. If such an alloy were available, it would help solve many of the most urgent problems facing the auto industry, including reduction in weight and improved engine emissions. Design criteria could be extended toward higher performance with such alloys.
One approach taken by the prior art is the use of ceramic fibers, ceramic whiskers, or ceramic particulates to reinforce a conventional aluminum alloy, the so-called ceramic reinforced metal matrix composites. Unfortunately, the material and processing costs of these approaches is much too high to be considered for large volume usage in massive engine components, such as pistons.
The approach of this invention is to eliminate reliance on the Si, Mg, Ni and Zn as strengthening, and instead utilize critical amounts of Mn, Zr and V, in addition to Cu, as strengthening agents. The level tolerated of Si, Fe or Mg, as an impurity, is as low as possible, and each of Ni and Zn is limited to 0.20% maximum.
A known commercial wrought aluminum alloy 2219 (not a casting alloy) is used in forging, extrusion, and rolling; it contains Mn, Zr, V and Cu as alloying agents but in a content regime that is low and thus does not obtain the physical properties of this invention. 2219 exhibits a UTS, after exposure to 500.degree. F. for 1000 hours, of about 29 ksi.
U.S. Pat. No. 2,706,680 (now expired) attempted to modify aluminum alloys to increase resistance to creep at high temperature (400.degree. F.) for jet engine use. This attempt essentially eliminated Si, Mg, Zn and Ni while substantially increasing Cu (up to 13%) and Mn (0.15-1.7%) over the 2219 alloy, accompanied by small amounts of V (0.05-0.2%) and Zr (0.05-0.3%) to improve creep resistance. Silicon was limited to 0.05-0.3% and iron limited to 0.05-0.5%. This chemistry resulted in a UTS value, after 1200 hours at 400.degree. F., that was not much different than the conventional 390 alloy. These physical properties fall short of the goal of this invention, namely, to provide an aluminum casting alloy that at 400.degree. F. after 1200 hours has a UTS of 42 ksi or greater, and after 1000 hours at 500.degree. F., a UTS of 33 ksi or greater.